The present invention relates to a wavelength conversion apparatus, and more particularly to a wavelength conversion apparatus with improved conversion efficiency, which can be easily adjusted to maximize the conversion efficiency, and which is insensitive to the polarization of signal light.
The conversion of light from one wavelength to another is necessary for purposes such as wavelength-division multiplexing in fiber-optic communications. Known wavelength conversion devices include bulk-element devices comprising non-linear optical crystals, integrated waveguide devices with a domain inversion grating formed in a semiconductor material, and integrated waveguide devices formed in non-linear optical materials, with or without a domain inversion grating. The methods of wavelength conversion employed in these devices include second-harmonic generation (SHG), sum-frequency generation (SFG), and difference-frequency generation (DFG).
A common problem in all of these devices is inadequate conversion efficiency. Efficiency is particularly low in bulk-element devices, because of dispersion.
In semiconductor waveguide devices with a domain inversion structure, the problem of low conversion efficiency is compounded by a complex and difficult fabrication process, including steps such as wafer bonding and crystal regrowth.
Waveguides formed in non-linear optical materials such as lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), and potassium titanyl phosphate (KTiOPO.sub.4, commonly abbreviated KTP) appear to offer the best hope for high conversion efficiency in a device of simple structure. The simplest structure directs pump light with a fundamental wavelength from a laser into a waveguide formed in a non-linear optical material. Non-linear optical effects convert part of the fundamental wave to a second harmonic wave having half the wavelength of the fundamental wave, but the conversion efficiency is inadequate when a low-power semiconductor laser diode is employed as the fundamental light source.
The conversion efficiency can be enhanced by placing wavelength-selective reflecting structures, such as multilayer reflective coatings or distributed Bragg reflector gratings, at each end of the waveguide, to confine the fundamental wave while transmitting the second harmonic wave. Such reflecting structures also reduce the amount of fundamental power coupled into the waveguide, however. Finding the reflectivities that optimize the conversion efficiency is a difficult design problem.
A further problem is that a stringent phase-matching or quasi-phase-matching condition is placed on the wavelength of the pump light. Since the laser that produces the pump light is separate from the wavelength conversion device, it is difficult to ensure that the phase-matching condition is satisfied. Careful design and tight manufacturing tolerances are necessary, and the wavelength conversion apparatus must operate in a strictly controlled environment.
If signal light is also introduced into the waveguide, interactions between the signal light and pump light can produce light with their sum or difference frequency. In this case, in addition to the problems described above, there is the problem of polarization sensitivity: the conversion efficiency depends on the plane of polarization of the signal light, which is inconvenient in many practical systems.